How to load an object from another scene in the bge?
You cannot add objects across scenes. Scenes are completely seperate. What you can do is you can use LibLoad to combine two blend files. So if you have your assets in one blend (assets.blend), you can use:  bge.logic.LibLoad(path_to_blend, 'Scene')
to merge the two scenes. If objects are on an inactive layer in assets.blend, then you can use addObject to add them when/where you want.

(note that the 'Scene' is to tell it to load whole scene, but does not tell it which. There is no way to tell LIbLoad which scene to load - it will do the one that the blend is saved with)